Organization of inputs to the dorsomedial nucleus of the hypothalamus: a reexamination with Fluorogold and PHAL in the rat.

Possible inputs to the DMH were studied first using the fluorescent retrograde tracer Fluorogold, and identified cell groups were then injected with the anterograde tracer PHAL to examine the distribution of labeled axons in and around the DMH. From this work, we conclude that the majority of inputs to the DMH arise in the hypothalamus, although there are a few significant projections from the telencephalon and brainstem. With few exceptions, each major nucleus and area of the hypothalamus provides inputs to the DMH. Telencephalic inputs arise mainly in the ventral subiculum, infralimbic area of the prefrontal cortex, lateral septal nucleus, and bed nuclei of the stria terminalis. The majority of brainstem inputs arise in the periaqueductal gray, parabrachial nucleus, and ventrolateral medulla. In addition, it now seems clear that inputs to the DMH use only a few discrete pathways. Descending inputs course through a periventricular pathway through the hypothalamic periventricular zone, a medial pathway that follows the medial corticohypothalamic tract, and a lateral pathway traveling through medial parts of the medial forebrain bundle. Ascending inputs arrive through a midbrain periventricular pathway that travels adjacent to the cerebral aqueduct in the periaqueductal gray, and through a brainstem lateral pathway that travels through central and ventral midbrain tegmental fields and enters the hypothalamus, and then the DMH from more lateral parts of the medial forebrain bundle. The results are discussed in relation to evidence for involvement of the DMH in ingestive behavior, and diurnal and stress-induced corticosterone secretion.